1. Field of the Invention
The present invention concerns friction clutches, notably for motor vehicles.
It relates more particularly to friction clutches having a wear take-up device.
2. Description of the Related Art
As is known, a friction clutch has at least one friction disc provided at its external periphery with friction linings, possibly divided, intended to be clamped between a flywheel, forming a reaction plate, and a thrust plate.
The flywheel is formed so as to be connected with respect to rotation to a first shaft, whilst the friction disc is formed so as to be connected with respect to rotation to a second shaft.
In the case of an application for motor vehicles the first shaft is a driving shaft, whilst the second shaft is a driven shaft, namely the input shaft of a movement transmission box, usually the input shaft of the gearbox. The flywheel, possibly in two parts in order, for example, to form a double torsion damping flywheel, is fixed centrally to the engine crankshaft by means of fixing members, such as screws, whilst the friction disc conventionally has a central hub fluted internally in order to connect it with respect to rotation to the said input shaft. This hub is rigidly or elastically coupled to the friction linings.
The thrust plate is connected with respect to rotation, with axial freedom of movement, to the flywheel and has at its front a friction face for the friction linings, whilst the flywheel has on its back a friction face for the said linings.
Axially acting elastic means bear on a cover and act on the rear face of the thrust plate in order to clamp the friction linings between the friction faces of the flywheel and of the thrust plate.
These elastic means are therefore engagement means tending to bring the thrust plate close to the flywheel in order to clamp the friction linings.
The cover is hollow in shape and is formed at its external periphery so as to be fixed to the flywheel by means of fixing means.
These fixing means can be of any type and can consist for example of welding and/or crimping means. Usually the thrust plate forms part of a unitary assembly, referred to as a engagement mechanism, including the thrust plate, the engagement means and the cover.
This unitary assembly also includes disengagement means associated with the engagement means in order to counteract the action of the engagement means. The thrust plate is in this case connected with respect to rotation, with axial freedom of movement, to the flywheel by means of the first means of connection with respect to rotation. For example, the thrust plate has at its external periphery, projecting radially, lugs, forming tenons, engaged in grooves, forming mortises, provided axially in an axially oriented cylindrical skirt which the cover has at its external periphery. This skirt is connected, at one of its axial ends, to a base with a central hole and oriented transversely. The base is directed radially inwards. At its other axial end, the skirt is fixed to the flywheel by means of the aforementioned fixing means. Thus the thrust plate is connected with respect to rotation by means of a connection through cooperation of shape of the mortise and tenon type.
Conventionally, the means of connecting with respect to rotation, with axial freedom of movement, consist of elastic tongues elongated circumferentially in the form of an arc of a circle or of tangential orientation. These tongues, distributed in several groups of one or more superimposed tongues, are fixed at one of their ends to the thrust plate, for example to radial lugs thereon, and at the other end to the cover, for example to a radial area thereon.
The tongues constitute return means and provide return of the thrust plate in the direction of the base of the cover in order to disengage the clutch, that is to say to release the friction linings of the friction disc so that they do not come to brush against the friction faces of the flywheel and of the thrust plate.
The engagement means can consist of a plurality of coil springs or an axially acting elastic washer of the Belleville washer type, whilst the disengagement means can consist of a plurality of disengagement levers acting on the Belleville washer in order to modify, in a controlled fashion, the clamping action exerted directly or indirectly by this Belleville washer on the thrust plate. Usually the engagement and disengagement means belong to one and the same part referred to as a diaphragm, having a peripheral part, in the form of a Belleville washer, extended radially inwards by a central part divided into radial fingers by slots. The radial fingers form the disengagement levers and therefore the disengagement means, whilst the axially acting Belleville washer forms the engagement means. As a variant, the disengagement means, can consist of a false diaphragm, that is to say a diaphragm whose peripheral part exerts a low, or even nil, elastic load. This can be produced by providing this peripheral part with radial slots. The false diaphragm can act on one or for example two Belleville washers mounted in series.
Whatever the case, by acting, by drawing or pushing, using a clutch release bearing on the inner end of the disengagement means, the said disengagement means are caused to pivot or tilt so that they act on the engagement means in order to reduce, and then to cancel out, the clamping load (the force) exerted by the said engagement means on the thrust plate. The friction linings are then no longer clamped between the thrust plate and the flywheel, so that the torque is no longer transmitted from the first shaft to the second shaft.
It should be noted that the elastic tongues, through their return action, separate the thrust plate from the friction linings in order to release the latter. The clutch is then disengaged or declutched. Normally the friction linings are clamped as mentioned above by virtue of the engagement means. The clutch is therefore normally engaged.
During the service life of the clutch the friction linings of the friction disc and to a lesser extent the friction faces of the flywheel and of the thrust plate wear. As a result the thrust plate moves closer to the flywheel. In the clutch engaged position, the load exerted by the engagement means therefore varies with the said wear.
The same usually applies to the position of the inner end of the disengagement means on which the clutch release bearing acts.
In order to prevent this, it has already been proposed to provide the friction clutch with a device for compensating for the wear notably on the friction linings and friction faces. This device, hereinafter referred to as the wear take-up device, makes it possible, for example in the case of a friction clutch provided with a diaphragm, to have, during the service life of the friction clutch, for the clutch engaged position, roughly the same position for the inner end of the diaphragm fingers and a roughly identical load exerted by the diaphragm on the thrust plate and friction linings.
The characteristic curve of the clutch release bearing remains roughly identical (the load exerted on the inner end of the diaphragm fingers according to the travel).
To do this, the wear take-up device has ramp means carried partly by a first piece fixed with respect to rotation and movable axially and partly by a second piece movable with respect to rotation and fixed axially. There is also provided a release sensitive to the state of wear at least of the friction linings in order to control the relative movement between the said pieces.
Usually the first piece is formed by the main part of the thrust plate and the second piece by the support offered by the thrust plate to the engagement means. For more information reference should be made to the document FR-A-2 426 834.
Normally (with the clutch in the engaged position) the second piece is clamped between the main part of the thrust plate and the engagement means, such as the Belleville washer of the diaphragm.
No rotation of this second piece is therefore possible. In the case of wear notably on the friction linings the release, sensitive to the variation in position of the thrust plate following the wear on the friction linings, makes it possible to release the second piece, which thus can turn during the disengagement manoeuvre of the clutch or when the clutch is in the disengaged position because the load exerted by the engagement means is small or nil. Wear on the friction linings therefore gives rise to an increase in the thickness of the thrust plate between its friction face and the support which it offers to the engagement means.
As a result the thrust plate is modified. To avoid modifying the thrust plate, it can be thought of adopting a solution described in the document FR-A-2 722 852 (FIGS. 1 to 6).
This solution results in locating the ramp means within the flywheel and in two parts.
One of the parts is in the form of a thrust plate offering a friction face for the friction lining concerned, whilst the other part consists of a support, transversely oriented, formed so as to be fixed to the first shaft.
Second means of connecting with respect of rotation, allowing an axial movement, act between the reaction plate and the support so that the said plate can move axially.
The reaction plate is shaped so as to carry, at its internal periphery, a part of the ramp means, whilst the support rotatably carries the other part of the ramp means in association with circumferentially acting locking means.
It may be desirable to simplify this type of solution to have a more certain and more reliable functioning of the wear take-up device.
The object of the present invention is to meet this requirement economically.
Therefore an aim of the invention is to create a friction clutch provided with a thrust plate of simple form, whilst simplifying the wear take-up device acting between the two parts of the flywheel.
Another aim of the invention is to make the functioning of the wear take-up device more certain and reliable whilst simplifying the said device.